1. Field of the Invention
The invention relates to a method of surface treatment for plaster articles to improve wear and water resistance and article of manufacture.
2. Description of the Prior Art
Gypsum products, particularly gypsum board, and plaster articles of manufacture, in which the binder is typically gypsum (various forms of calcium sulphate), are widely used in commercial, industrial and residential applications. While gypsum wall board provides outstanding fire resistance and relatively low-cost, it has the disability of not being water resistant. Plaster articles, such as industrial casts, have the distinct ability to be molded into an infinite variety of shapes. However, such cast plaster articles do not offer significant wear resistance, and are additionally susceptible to moisture.
As disclosed in the prior art, when gypsum wall board is immersed in water at room temperature the lack of water resistance is readily demonstrated by the fact that in a short time the gypsum core has absorbed a considerable amount of water. Reports from numerous tests demonstrate that when a two inch cube of gypsum board core material is immersed in water at about 70.degree. F. (21.degree. C.), the material shows a water absorption of greater than 60% during immersion for five minutes. The prior art discloses numerous attempts to enhance water resistance and concomitantly wear resistance. Predominant among such previous attempts are those involved with the application of a water resistant material to a gypsum or plaster slurry prior to setting.
The prior art discloses that the introduction of a water resistant material into the slurry includes such compositions as metallic soaps, asphalts, waxes, and resins. A well known method is the incorporation of paraffin wax and asphalt, or other fusible water-insoluble organic substances, into a gypsum mix. First this waxy material is melted and then dispersed into the wet gypsum mix by spraying the hot molten mass into the aqueous gypsum dispersion. While obtaining water resistance, such related processes require a rather costly and large amount of materials without actually obtaining a drastic amount of water resistance or wear resistance. It would be desirable to use a comparatively small amount of water proofing and wear resistant material to obtain desired results.
Other well know techniques involve utilizing a mixture of paraffin wax and asphalt, in definite proportions, on the order of from about 50% to about 15% (dry weight), incorporated in the form of an emulsion with a wet plastic gypsum mass. The finished dry gypsum wall board will possess a degree of water resistance which is greatly out of proportion to the degree of water resistance obtained if one uses the same paraffin wax and asphalt materials without emulsification. Related teachings disclose the addition of a composition of asphalt, paraffin wax and potassium sulphate. While significant water resistance is obtained, it is necessary to include the added materials to the entire gypsum slurry. A certain amount of unnecessary addition is thus required, such as at the interior of the core, where water resistance is not as critical as at the exterior faces of the board, or molded article.
Another method, in line with the last mentioned procedures, utilizes a composition of asphalt, paraffin wax and a substance selected from the group consisting of alkali and alkaline earth aluminates and Portland cement. Similarly asphalt and paraffin wax compositions are utilized wherein a blend of oil-soluble alkali salt of mahogany sulfonic acid is provided.
In taking a different approach, others have utilized polyvinyl alcohol for both sag resistance and the sizing of paper coatings. In this regard some have used polyvinyl alcohol and an asphalt-wax emulsion but require that a significantly large percentage of the weight be polyvinyl alcohol. Since the cost of polyvinyl alcohol may be prohibitive, the end result--water resistance--may be extremely costly. Additionally, utilization of asphalt-wax emulsions for treating the gypsum core do not provide significant wear resistance.
Further water resistant additives, which have been effective to various degrees, include the following: mixtures of wax and/or asphalt, and also corn flour and potassium permanganate; water-insoluble thermoplastic organic materials, such as petroleum and natural asphalt; coal tar and thermoplastic synthetic resins such as polyvinyl acetate, polyvinyl chloride and a copolymer of vinyl acetate and vinyl chloride; a mixture of an alkali metal rosin soap, a water soluble alkaline earth metal salt and residual fuel oil; petroleum wax mixtures in the form of an emulsion and either residual fuel oil, pine tar or coal tar; other mixtures utilizing residual fuel oil and rosin; aromatic isocyanates and diisocyanates; and, organohydrogenpolysiloxanes. The consistency of water resistance provided by such techniques has not been established to provide uniform results. Moreover, utilizing wax-asphalt emulsions in gypsum compositions has a tendency to retard the hydration of the gypsum. With the well known use of gypsum set accelerators, additional amounts of accelerators are required to overcome the retardation by the wax-asphalt emulsion. The use of the additional amount to offset the retarding effect of the wax-asphalt emulsion results in a decrease of water resistancy.
Additional utilizations of resins for inclusion in a gypsum or plaster slurry include hydrophobic natural, synthetic and modified varieties, for example, ester gums, maleic anhydride modified rosin, phenol formaldehyde modified rosin, copolymers of styrene, vinyltoluene butadiene copolymers, polyvinyl chloride, polyvinyl acetate, acrylic copolymers, and petroleum and coal tar hydrocarbon resins. Attempts at reducing the amount of asphalt and wax include the use of a small amount of a borate compound having an anion comprising boron and oxygen, such as borax. It has been determined that the use of a small amount of a borate compound permits a smaller amount of the relatively expensive polyvinyl alcohol to be used without a reduction in the water resistance of the final product.
Recent improvements in the utilization of paraffin emulsions set forth the preparation of this emulsion by emulsifying a paraffin hydrocarbon having a melting point of 104.degree.-176.degree. F. (40.degree.-80.degree. C.) and an oxidized paraffin having an acid value of 10-70 at a ratio of from 97:3 to 50:50 by weight, respectively, in the presence of a water-soluble alkali compound. Such procedures have been utilized to maintain the adhesiveness between a gypsum core and paper coating and maintain the conventional physical properties of the gypsum board.
While the foregoing illustrates the extensive prior practices with regard to mixing various compositions with a gypsum or plaster slurry, relatively few surface coating or post-treatments for plaster articles are disclosed. In very early attempts, surface coating techniques utilized total immersion of a plaster article in a bath prepared by melting wax, such as Carnauba wax. Such techniques are time consuming and costly. The amount of water resistancy produced was inexact and no degree of certainty could be predicted. Moreover, an inordinate amount of wax was used due to the inherent wastefulness of such total immersion techniques. It is also problematic to not significantly heat a plaster article after it has been dried since re-calcination can occur at high temperatures when exposure is prolonged.
While many prior art attempts have striven toward enhanced water resistance for gypsum board and plaster cast articles, the attainment of wear resistance, as well as water resistance, has been minimally treated. The use of a surface coating for treating plaster articles would be highly desirable for providing both wear resistance and water repellency. Such would be desirable in that the surface protection would require only a treatment at the exterior portion of a plaster article rather than the costly, and sometimes deleterious, utilization of a composition introduced throughout the slurry prior to setting.
It is accordingly a primary goal of this invention to provide a method of surface treatment for plaster articles to improve wear and water resistance.
It is additionally an important goal of this invention to provide a method of surface treatment utilizing a wax material in a relatively small amount and thus alleviating the costly need for inclusion of a composition throughout the plaster slurry.
It is an allied object of the invention to provide both wear resistance and water repellency for plaster articles in a method of treatment subsequent to the article being dried to remove excess water.
It is also a concomitant object of the invention to utilize a wax, which has a low melt point, for treating an exterior surface of a plaster article, wherein post-heating of the plaster article at elevated temperatures would not be required such that re-calcination can be avoided.
Accordingly, it is a goal of the invention to utilize a wax in a surface treatment for plaster articles having a melt point which is high enough that at elevated ambient room temperatures, the surface coating retains its hardness.
It is a related object of the invention to utilize a wax, by applying the wax in molten form onto an exterior surface of a plaster article, such that the wax can substantially fill exterior pores and irregularities to thereby provide a surface coating which is wear and water resistant.